When a polymer electrolyte fuel cell (PDFC) generates power, if the membrane electrolyte dries out, the ionic conductivity falls, so a resistance overvoltage increases and the power generating efficiency falls. Conversely, if the water in a gas diffusion layer situated on both sides of the membrane electrolyte is excessive, it interferes with the diffusion of reactive gases to the electrodes of the fuel cell, so a diffusion overvoltage increases and the power generating efficiency again falls. Herein, the reactive gas is air for the cathode and hydrogen for the anode. In the air, only the oxygen is used for reaction.
To supply a suitable amount of water to the membrane electrolyte, Tokkai 2000-323159 published by the Japanese Patent Office in 2000 discloses a frame-shaped water-absorbing sheet surrounding the cathode and joined to the membrane electrolyte. At the cathode, water is produced by the reaction of hydrogen ions which have passed through the membrane electrolyte and oxygen in the air. When the water-absorbing sheet adsorbs and holds this water, the membrane electrolyte and air supplied to the fuel cell from outside is humidified. By humidifying the membrane electrolyte, water which has passed through the membrane electrolyte also humidifies hydrogen supplied to the anode on the other side of the membrane electrolyte.